Semiconductor device is constructed by providing n-type and/or p-type regions in a semiconductor substrate to form a semiconductor circuit and connecting a metal wiring material to the semiconductor region, or the n-type or p-type region, which is required for an electrical contact with exterior.
For instance, an explanatory cross sectional view of a npn bipolar transistor is shown in FIG. 6. In FIG. 6, an impurity such as boron is diffused into a n-type silicon semiconductor substrate 1 to form a p-type base region 2, and further another impurity such as phosphorus is diffused in a part of the base region 2 to form a n-type emitter region 3. Next, the whole surface of the semiconductor substrate 1 is covered with a protective film 4 made of, for instance, silicon dioxide wherein the contact formation regions for the base and emitter regions 2 and 3 are then exposed to form contact holes 5 and 6. Subsequently a metal wiring material such as aluminum is deposited over the whole surface of the substrate by sputtering or a like method, followed by etching to form a wiring pattern on the protective film 4. As a result, the contact holes 5 and 6 are refilled with the metal wiring material which comes into direct contact with the base and emitter regions 2 and 3, thereby forming a base electrode 7 and an emitter electrode 8. The n-type semiconductor crystal layer of the semiconductor substrate 1 operates as a collector region for which a collector electrode 9 is provided by depositing a metal material such as gold on the lower side of the substrate 1.
In an integrated circuit, for instance, if aluminum as a surface electrode metal is deposited directly on a semiconductor region, incomplete adhesion therebetween due to the difference of the materials undesirably increases the contact resistance, so as to cause degradation of device characteristics. In order to reduce the contact resistance, there is generally employed is a method wherein the semiconductor region in contact formation regions is selectively and heavily doped with an impurity.
However, in a semiconductor device such as a bipolar IC, the depth of impurity diffusion is generally small, hence, the base resistance and emitter resistance are too small to operate as a buffering resistance. Accordingly, current tends to be concentrated when large amount of current is applied to the device, which raises a problem of a narrow SOA and, hence a decreased secondary breakdown strength.
If it is made to broaden the SOA of the conventional semiconductor device, the depth of impurity diffusion in the base and emitter regions 2 and 3 would need to be enlarged so as to increase the resistance thereof. This raises a problem of an increase in a diffusion time so as to increase the time for producing, hence in the production cost.